The present invention relates to superabsorbent polymers having enhanced swelling characteristics. More particularly, the present invention relates to an absorbent composition comprising an ionic superabsorbent polymer material and an absorbency-enhancing additive having a dipole moment greater than water which significantly increases the swelling characteristics of ionic superabsorbent polymers in the presence of salt-containing solutions such as urine, menses, blood and other body exudates.
The use of superabsorbent polymers such as ionic superabsorbent polymers is standard throughout the absorbent products industry. The absorbent core of many absorbent articles, such as diapers, sanitary napkins, training pants, and incontinence products, is typically comprised of one or more layers of cellulose fluff pulp, intermixed with superabsorbent polymers. Other layers are often used in combination with the absorbent core depending upon the intended application of the product.
Ionic superabsorbent polymers, also commonly referred to as ionic hydrogels or ionic hydrocolloids, are typically cross-linked ionic polymers that are able to absorb an amount of pure water equal to at least ten times their dry weight and retain the pure water under a moderate external pressure. Ionic superabsorbent polymers can be anionic in nature (e.g., acrylate based or sulfonate based), or can be cationic in nature (e.g., a partly neutralized polyamine), and as such can either have positive or negative charges along the backbone of the polymer structure. Ionic superabsorbent polymers are electrically charged in solution because various groups attached to the polymer chain easily become ionic. Examples of groups which can become electrically charged in ionic superabsorbent polymers include carboxylate groups and amine groups.
Liquid absorbed by a superabsorbent polymer is taken directly into the molecular structure itself, and is not simply contained in pores or openings in the material from which it could be easily expressed by the application of pressure. The preferred commercially available ionic superabsorbent polymers are generally crosslinked polyacrylates such as poly(acrylic acid) or acrylic acid grafted onto starch. The carboxyl functionality on the polymer backbone is partially neutralized with sodium or potassium hydroxide. Some ionic superabsorbent polymers are formed by graft polymerizing acrylonitrile onto gelatinized starch followed by hydrolysis of the polyacrylonitrile to poly(acrylic acid-co-acrylamide).
Although ionic superabsorbent polymers have the ability to absorb many times their weight of pure water, when contacted with aqueous salt solutions, their ability to effectively absorb liquid is generally reduced by more than a factor of five, or even more, depending upon the ionic strength of the salt solution. The degradation of absorption capacity suffered by ionic polymers in salt solutions is believed to be due to a collapse of the counterion atmosphere surrounding the ionic backbone of the polymer chains. The counterion atmosphere is made up of ions of opposite charge to the charges along the backbone of the ionic polymer and are present in the salt solution (e.g., sodium or potassium cations surrounding the carboxylate anions distributed along the backbone of a polyacrylate anionic polymer). As the concentration of ions in the salt solution contacting the superabsorbent ionic polymer increases, the ion concentration gradient in the liquid phase from the exterior to the interior of the polymer begins to decrease and the counterion atmosphere thickness, typically referred to as the Debye thickness, can be reduced from about 20 nanometers (in pure water) to about 1 nanometer or less. When the counterion atmosphere is highly extended (i.e., not significantly collapsed around the polymer backbone as when pure water is absorbed), the counterions are much more osmotically active and therefore promote a much higher degree of liquid absorbency. However, when the ion concentration in the absorbed liquid increases, the counterion atmosphere collapses and absorption capacity is diminished.
Hansen et al., in U.S. Pat. No. 5,998,032, disclose a process for enhancing the blood absorbence properties of superabsorbent materials by combining an enhancing agent with the superabsorbent particles. The enhancing agent includes functionalities that allow the enhancing agent to hydrogen bond to the superabsorbent material. A suitable enhancing agent, such as glycine, is present in an amount up to 10% of the total weight of the superabsorbent material and enhancing agent. Although Hansen et al. do disclose a slight improvement in the blood absorbent properties of superabsorbent materials, a need continues to exist in the art for improved superabsorbent materials capable of absorbing increased amounts of salt-containing body exudates.